Asymmetric septal hypertrophy and myocardial fiber disarray. Features of normal, developing, and malformed hearts.

The specificity and significance of the asymmetric septal hypertrophy (ASH) and myocardial fiber disarray of idiopathic hypertrophic subaortic stenosis (IHSS) is uncertain. To examine this we studied 215 hearts, including normal embryos, fetuses, children, and adults; and hearts with congenital and acquired disease. Disproportionate septal thickening was present in all embryos and in some abnormal hearts, particularly those with severe right ventricular hypertrophy due to congenital malformations. Some myocardial fiber disarray was present in all hearts at the junctions of interventricular septum and ventricular free wall. In hearts with semilunar valve atresia with intact ventricular septums, and in the infundibulum of some with tetralogy of Fallot, however, extensive fiber disarray was present. Thus, ASH occurs in the normal developing heart and in some malformed hearts with RVH; marked muscle fiber disarray may occur in certain congenital lesions with abnormal systolic contraction. Neither morphologic finding independently or in combination is pathognomonic of idiopathic hypertrophic subaortic stenosis.